Mycoherbicides are delivered to the weed target species as spores, mycelia, or other living propagules of the fungus. Spores of various fungi may be classified as hydrophobic or hydrophilic, and this property could determine the nature of various mycoherbicide formulations. As such, a hydrophobic spore, e.g., Alternaria cassiae, requires the addition of a surfactant to an aqueous spore suspension to facilitate dispersion. See U.S. Pat. No. 4,390,360.
In addition to being utilized in an actual mycoherbicidal concentrate, surfactants are also used in spray solutions to facilitate mycoherbicide penetration into the plant. The addition of a surfactant to either the concentrate of the actual spray solution can enhance or retard herbicidal efficacy, or it may have no effect. The effect of surfactants on mycoherbicidal efficacy is complicated by the fact that the surfactant may also affect the infection, virulence, germination or other property of the living mycoherbicidal propagule.
Regardless of the type of mycoherbicidal concentrate formulation, a wetting agent will be used in the spray solution of postemergence mycoherbicides to achieve even distribution of the mycoherbicide over the leaf surface. Wetting agents often comprise from 0.02% to 2% v/v of the aqueous spray solution.
It has been documented that spore suspensions in sterile water, fluorochemicals, oil or talc can be atomized onto leaf surfaces to inoculate plants (Dhingra, O. D. and Sinclair, J. B. [1985] Basic Plant Pathology Methods, CRC Press, Inc. Boca Raton, FL, p. 137). The use of non-phytotoxic oils as a suspending medium is also documented (Dhingra and Sinclair); however, some conidia may lose infectivity when suspended in oil (Bushnell, W. R. and Rowell, J. B. [1967] Plant Dis. Rep. 51: 447). It has been recently stated by Dhingra and Sinclair that oils maintain a uniform suspension of spores more easily than does water, and no surfactant needs to be added.
The wettability of the spore surface has been described as being an extremely variable physical property (Hawker, L. E. and Madelin, M. F. (1976) in The Fungal Spore, Form and Function, D. J. Weber and W. M. Hess, eds. John Wiley and Sons, New York, pp. 1-70), and variation has been observed among species of the same genus. The presence of surface lipids in spores of Alternaria tenuis, Botrytis fabae, and Neurospora crassa probably contributes to their hydrophobic nature, although hydrophobicity is also noted in Penicillium sp., Aspergillus sp. and Erisyphe sp. which lack surface lipids. Verticillium albo-atrum and Nectria galligena also lack surface lipids (Fisher, D. J., Holloway, P. J. and Richmond, D. V. [1972] J. Gen. Microbiol. 72: 71-78) but they are hydrophilic. Thus, hydrophobicity/hydrophilicity of the spore surface is not a predictable variable. Yet, it is very important to understand this property to optimize inoculation of a plant with a mycoherbicide. For example, a hydrophilic, water-dispersed spore is not very easily dispersed in an oil. Conversely, a hydrophobic spore would not be very easily dispersed in water without the aid of a surfactant.